# These settings were set by the catalyst build script that automatically
# built this stage.
# Please consult /usr/share/portage/config/make.conf.example for a more
# detailed example.
# !!Do NOT change anything except for 'user config'!!
CFLAGS="-O2 -pipe -march=core2"
CXXFLAGS="${CFLAGS}"

# WARNING: Changing your CHOST is not something that should be done lightly.
# Please consult http://www.gentoo.org/doc/en/change-chost.xml before changing.
CHOST="x86_64-pc-linux-gnu"

I like to use -Os over -O2 personally, the minor performance win for -O2 is not worth the extra memory and hard disk consumption, and the cache would not necessarly be more efficiently used.

I used to have -march=core2 but I don't know if I should switch to =native.

Code:

# These settings were set by the catalyst build script that automatically
# built this stage.
# Please consult /usr/share/portage/config/make.conf.example for a more
# detailed example.
CFLAGS="-Os -pipe -march=core2"
CXXFLAGS="${CFLAGS}"
# WARNING: Changing your CHOST is not something that should be done lightly.
# Please consult http://www.gentoo.org/doc/en/change-chost.xml before changing.
CHOST="x86_64-pc-linux-gnu"

# These settings were set by the catalyst build script that automatically
# built this stage.
# Please consult /usr/share/portage/config/make.conf.example for a more
# detailed example.
CFLAGS="-march=native -O2 -pipe"
CXXFLAGS="${CFLAGS}"
# WARNING: Changing your CHOST is not something that should be done lightly.
# Please consult http://www.gentoo.org/doc/en/change-chost.xml before changing.
CHOST="x86_64-pc-linux-gnu"
# These are the USE flags that were used in addition to what is provided by the
# profile used for building.
MAKEOPTS="-j5"

Side note: how do I define USE_EXPAND'ed flags like INPUT_DEVICES= in package.use files?

I am not sure if I get what you mean...
Do you setting USE flags for particular packages... that have those <SOMETHING_LIST> environment variables?
Easy, `input_devices_<flag>' USE flag(s) for INPUT_DEVICES flags and so on._________________home/:mkinitramfs-ll/:supervision(-scripts)/:e-gtk-theme/:(bar-)overlay/